This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The overall, long-term goal of our research is to examine how aging alters skeletal muscle function in humans at the molecular level. Age-related reductions in muscle mass and performance, or sarcopenia, decrease a person's ability to perform everyday tasks and increase their risk of falling and fall-related injuries. A key predictor of this decrease in functional capacity is a reduction in muscle power (force x velocity). Although studies have shown that single muscle fiber force production and contractile velocity are reduced with age, no reported study has examined the molecular basis of the age-related decrements in single fiber muscle performance. In order to create an environment as similar to in vivo as possible in our skinned single skeletal muscle fiber experiments, we propose to use an osmotic compressor, Dextran T500, to return the swollen skinned lattice spacing back to in vivo values. Use of the BioCAT beam line allowed us to use x-ray diffraction to measure the lattice spacing of unskinned fibers, which would approximate in vivo conditions, as well as skinned fibers compressed with different concentrations of Dextran T500 to determine the amount of osmotic compression necessary to return the swollen skinned fibers to in vivo lattice spacing values. Based on these x-ray results, we will perform our single fiber measurements at the University of Vermont using the concentration of Dextran T500 necessary to return the lattice spacing to in vivo values.